This is a good way to get around the towns and cities, and is normally quite cheap, with frequent services in major cities. Using buses to travel long distances is also a good, inexpensive option. Bus links within the United-Kingdom are generally quite convenient.
The main bus company that serves the United Kingdom is National Express. Their busses are of very high standard and typically run on schedule.
On the down side, as in most major cities around the globe, large bus stations have always been a favourite target for pick pockets and luggage thieves. So when you use a bus station keep an eye on your luggage at all times. Do not be tempted to go into one of the many shops that often surround stops and leave your luggage unattended, if even for a minute.
Busses can also get very crowded on long distance trips. This being said, if your journey is going to be more than an hour it is a good idea to book your ticket in advance if the bus company allows this.